Numerous attempts to improve the wicking properties and wet strength properties of highly absorbent web structures have been described in the art.
Sigl, et al., U.S. Pat. No. 4,049,764, issued Sept. 20, 1977, disclose an extrusion process for the formation of cellulose fiber webs. Highly absorbent and rapidly wicking filaments and web structures are formed when the extrudate is treated prior to extrusion with a solvent like acetone.
U.S. Pat. No. 4,307,143, issued Dec. 22, 1981, to Meitner, discloses wetting agents suitable for making polypropylene webs oil and water absorbent. The wetting agents are selected from the group consisting of dioctylester of sodium sulfosuccinic acid and isooctyl phenylpolyethoxy ethanol.
Silica has been proposed for modifying the properties of fibers and sheet like materials. For example, U.S. Pat. No. 2,443,512, issued June 15, 1948 to Powers, discloses the use of silica sols for improving the manipulative characteristics of textile fibers. The fibers are simply dipped in a silica aquasol and subsequently dried. This method of treatment is not capable of permanently modifying the fiber properties, but the silica particles stay on the fiber surface long enough to improve breaking strength and reduce slippage during subsequent processing (i.e., weaving). Colloidal silica has further been proposed for treating non-fibrous, non-porous cellulose sheets to prevent them from sticking together (U.S. Pat. No. 2,658,835, issued Nov. 10, 1953 to Wymbs). The sheets are simply soaked in a bath containing colloidal silica, and subsequently dried. A colloidal silica deposit in wound dressings may act to improve the strike through characteristics of the pad, as described in U.S. Pat. No. 3,381,688, issued May 7, 1968 to Satas. Since the silica is simply sprayed on, its substantivity to the absorbent web is poor. Application of the silica is therefore limited to these parts of the dressing which do not come into contact with the skin or the wound.
For applications where substantivity of the silica coating is indispensable, relatively complicated methods of coating have been developed. For example, where silica is used as a binder in fibrous refractory materials it has been suggested to use colloidal silica particles having a positive surface charge, to flocculate the silica onto the refractory fibers by negatively charged clay minerals, to subsequently impregnate the material with negatively charged colloidal silica, and bake at 200.degree. C. or higher (U.S. Pat. No. 3,775,141, issued Nov. 27, 1973 to Weidman). Baking is, of course, not possible where the fiber or sheet is a polymer like polypropylene. Substantivity of a silica or silicate comprising hydrophilic coating for polyester may be achieved by the use of complicated chemical mixtures. See, for example, U.S. Pat. No. 3,720,539, issued Mar. 13, 1973 to Seibel, et al., which discloses pretreatment of the polyester article with an aqueous solution of trichloroacetic acid, polyvinyl alcohol and a cleansing agent, and subsequent treatment with a solution comprising aluminum silicate, polyvinyl alcohol, polyvinyl acetate and a melamine-formaldehyde resin. As exemplified by the references cited herein, prior art methods of forming substantive silica coatings involve complex processes. With the process of the present invention silica coated fibers can be made by a simple process. Absorbent webs made of such fibers have excellent absorption and wicking properties.